1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
In recent years, two-dimensional image sensors serving as cameras and the likes mounted on portable devices such as digital cameras and portable telephone devices are becoming higher in quality and performance, and lower in price. Such two-dimensional image sensors are normally used for taking photographs. In line with today's trend of higher image quality and higher performance, however, the two-dimensional image sensors might be used to pick up images of documents such as paper documents, newspapers, and magazines, and store the text information contained in the document images as document image data.
Document image data that is generated by taking an image of a document using a two-dimensional image sensor and is formed with several sets of pixel data should preferably present easy-to-read text information contained in each document as an image pickup object. However, a camera formed with a two-dimensional image sensor or the like is not designed to set conditions such as the position of the user, the lighting environment, and the degree of distortion in the document as the image pickup object. Therefore, the generated document image data might not be easy to read.
To counter this problem, a technique for measuring and correcting the degree of distortion of each document as an image pickup object has been suggested.
For example, an image reading device disclosed in JP-A-10-150532 is designed to increase the reading accuracy about one-point-bound documents, and widen the range of use. The image reading device determines whether a document placed on a platen is a one-point-bound document that is bound at one corner and has one or more pages turned over. Based on an image picked up from above and an image picked up from a side with the use of an image pickup unit of an optical system, the image reading device measures the curvature of the document face that is the upper face of a paper sheet in the one-point-bound document. The image reading device then corrects the read image of the document face in accordance with the measured curvature.
An image reading device disclosed in Japanese Patent No. 3433624 corrects distortions in a read image caused due to a partially hidden document face, and increases the read accuracy when reading a one-point-bound document. The height distribution of the document face of a document placed on the platen is measured in the main scan direction and the sub scan direction, from an image picked up from above and an image picked up from a side with the use of an image pickup unit of an optical system or the like. The degree of curvature in the document face in each direction can be determined through the measurement. Based on the degree of curvature, the image reading device determines whether the document placed on the platen is a one-point-bound document that has sheets of paper bound at one corner and has one or more of the sheets turned over. Based on the height distribution of the document face in the main scan direction and the sub scan direction, the image reading device detects a sheet overlapping region in the document face that is the upper face of a paper sheet of the one-point-bound document. The image reading device then performs a correction to erase the portions corresponding to the overlapping region among the read images of the document face. The image reading device disclosed in Japanese patent No. 3433624 also uses an AF mechanism of an optical system or the like to make a focus adjustment when scanning the document in accordance with the height distribution in the document face. In this manner, the image reading device performs a height interpolation.
A document reading device disclosed in Japanese patent No. 3175250 corrects row-direction curvature. To do so, the document reading device reads from above a document facing upward, and detects the document shape by a triangular ranging method using a curvature sensor board or the like that includes an IR light emitting device and a light receiving device of an optical system. In accordance with the shape detection output, the document reading device corrects the row-direction curvature of the read document.
In the conventional techniques (JP-A-10-150532, Japanese Patent No. 3433624 and No. 3175250 and the like), the degree of distortion of a document as an image pickup object is measured and corrected with the use of an optical system. However, when the type of document is determined or the degree of distortion of the document is measured, it is necessary to prepare an image picked up from above and an image picked up from a side. As a result, the work load in the image processing becomes larger, and the efficiency becomes poorer. Also, in the conventional techniques (JP-A-10-150532, Japanese Patent No. 3433624 and No. 3175250 and the like), there is a problem with correction accuracy, since a verifying operation is not performed to determine whether the degree of distortion of each document has been accurately measured. Also, in the conventional techniques (JP-A-10-150532, Japanese Patent No. 3433624 and No. 3175250 and the like), fixed tools and devices are required to measure the degree of distortion. As a result, the devices become large in size, and the portability and versatility become poorer.
More specifically, in the image reading devices according to JP-A-10-150532 and Japanese Patent No. 3433624, the type of document is determined, and the degree of distortion in the document placed on the platen is measured, based not only on an image of the document picked up from above with the use of an optical system (such as an image pickup unit) but also on an image of the document picked up from a side by projecting the image onto a projection plate extending along the rear end of the platen or a projection plate extending forward or backward in the right half of the platen. Therefore, there is the need to perform various operations, such as an operation to determine whether the image picked up from above contains an image of a staple, an operation to determine whether the document is a book-type document or a sheet-type document based on the degree of curvature (the height distribution) obtained from the image picked up from a side, and an operation to correct the distortions in the images so as to achieve a uniform height based on the height distribution obtained from the image picked up from a side. Because of these operations, the work load in the image processing becomes larger. Also, there is the problem that increasing the correction accuracy by verifying the measured height distribution of the document is not considered prior to the correcting operation. Since fixed tools (such as the platen and the projection plate) and devices (such as an image pickup unit and an AF mechanism) are required, the device becomes too large in size.
The document reading device according to Japanese Patent No. 3175250 uses an optical system (such as a curvature sensor board) to measure the height of the document face according to a triangular ranging method. By the triangular ranging method, a distance is measured by receiving light that is emitted from an IR light emitting device, is reflected by the document face, and is received by a light receiving device. The document reading device corrects the row-direction curvature of the document in accordance with shape data interpolated based on the measured height. Accordingly, a correcting operation can be performed based only on the image picked up from above. However, increasing the correction accuracy by verifying the row-direction curvature of the document measured by detecting the document shape is not considered prior to the correcting operation. Also, since fixed tools (such as a board having the platen and a photo sensor arranged thereon) and devices (such as a reading unit including a curvature sensor board, an IR light emitting device, and a light receiving device) are required, the device becomes too large in size.